(1) Field of the Invention
This invention relates to a process for preparing polyamides having a high degree of polymerization from diamines and dicarboxylic acids, and particularly to a novel continuous polymerization process for preparing wholly or even partially aromatic polyamides from aromatic diamines and aromatic dicarboxylic acid dihalides. Linear polyamides having a high degree of polymerization made from diamines and dicarboxylic acid dihalides, particularly aromatic diamines and aromatic dicarboxylic acid dihalides can be processed into fibers, films, coatings and other shaped articles.
(2) Description of Related Art
Heretofore, (a) an interfacial polycondensation method, (e.g. U.S. Pat. No. 3,006,899), and (b) a low temperature solution polycondensation method, (e.g. U.S. Pat. No. 3,063,966), have been known as methods of preparing aromatic polyamides from aromatic diamines and aromatic dicarboxylic acid halides. These methods are regarded as being superior to other methods of obtaining aromatic polyamides, such as (i) dehydration condensation by heat between aromatic diamines and aromatic dicarboxylic acids, or (ii) condensation by heat with removal of phenols between aromatic diamines and diphenyl esters of aromatic dicarboxylic acids. In the dehydration-condensation method, the condensation does not proceed smoothly even at temperatures above 200° C., and in most cases, heat decomposition occurs to form a colored product containing insoluble gel. In the condensation with phenyl esters method, the phenyl esters must be derived from the corresponding acid chloride, which is a better reactant to give polyamides, but makes the method less efficient. These condensation methods must also be carried out at high temperatures for long periods of time, whereas the first-mentioned two methods (a) and (b) may be carried out at room temperature or below and require only a few minutes for the reaction to complete.
However, the first-mentioned two methods, (a) interfacial polycondensation and (b) the low temperature solution polycondensation method have various disadvantages. It is known that in the interfacial polycondensation method (a), controlling the molecular weight of the product is very difficult. The degree of polymerization of the resulting polymer depends upon and is very susceptible to slight changes in non-stoichiometric conditions, such as the concentrations of reactants, rates of addition thereof, size of the reactor vessel, the speed of stirring, and the type of solvents to be used. Once reactants have been charged, it is impossible to adjust the degree of polymerization of the final polymer by an additional feed because the degree of polymerization is predetermined by a delicate combination of the reaction conditions such as mentioned above. Moreover, when a copolymer is to be prepared according to the method, the main chain structure of the copolymer cannot be controlled. It has also been found that if a large-sized reaction vessel is used in practice, a polymer having a high degree of polymerization is not obtainable by this method.
On the other hand, in the low temperature solution polycondensation method (b), the molecular weight control of the final polymer is rather easy. However, when high molecular weight products are sought the selection of solvents to be used is limited, and various difficulties are encountered, for instance, in the separation of the product from the solution and from the occurrence of side-reactions. In addition, the product is always present together with hydrohalic salts as the reaction by-product. The removal of the by-product requires a long and tedious process, such as precipitation-and-rinse, or a complicated treatment after shaping the product into an article. It is therefore difficult to obtain a product free from those salts and having sufficient heat stability.
Two patents, U.S. Pat. Nos. 3,640,970 and 4,009,154, disclose a two-step process for preparing aromatic polyamides by (1) reacting substantially equal molar amounts of diamine with a dicarboxylic acid dihalide in a polar, non-basic, inert organic liquid medium to form a prepolymer; and (2) contacting the prepolymer with an aqueous solution of water-soluble acid acceptor to complete the polymerization reaction. These patents are as follows:
U.S. Pat. No. 3,640,970 discloses a two-step precondensate process for preparing aromatic polyamides, which is sometimes referred to as the oligomer polymerization process. In the first step of the process an aromatic diamine is reacted with an aromatic dicarboxylic acid dihalide in a polar, non-basic, inert organic liquid medium under conditions such that a condensation product having a low degree of polymerization is formed. In a second step the organic liquid medium containing the condensation product is contacted with an aqueous solution of a water-soluble acid acceptor to form the polyamide product.
Ethers, ketones, sulfones, and halogenated hydrocarbons are suggested as being suitable solvents. Water-soluble acid acceptors suited for neutralizing by-product hydrohalide acid include inorganic and organic alkali-metal hydroxides, carbonates and bicarbonates as well as organic amines, e.g., triethylamine and triethylenediamine. Inherent viscosities for the product polyamides of 0.6 to 3 in sulfuric acid were deemed possible.
U.S. Pat. No. 4,009,154 discloses almost the same two-step precondensate process disclosed in the '970 patent, except that the water-soluble acid acceptor is limited to an aqueous slurry consisting of a dispersion in water of a sodium carbonate hydrate. It was recognized in the '154 patent that when one attempted to practice the process of the '970 patent with an organic liquid medium containing the aromatic diamines and aromatic dicarboxylic acid halides in high concentration in order to improve efficiency, only aromatic polyamides of a low degree of polymerization could be obtained.
The processes of U.S. Pat. Nos. 3,640,970 and 4,009,154 are not conducive to large-scale continuous operation. Either the reaction is run too slowly resulting in polyamides of a low degree of polymerization, or these processes require very large pumps, very vigorous stirring, and very large process relief valves. Operation in this manner is not efficient and has associated safety concerns.
Therefore, there exists a need in the art for a continuous process that is (1) capable of efficiently producing polyamides of a high degree of polymerization; and (2) does not suffer from limitations on the nature of the introduction of the acid acceptor.